Radiator-excited cavity for nqr detection

ABSTRACT

Nuclear quadrupole resonance measurement using two or more wire loop(s) within a space to define a portal, and driving the wire loop(s) with a baseband digital transmitter generating a chirped or stepped signal, to create a corresponding varying electromagnetic field within the portal. Coherent emissions reflected thereby are detected through a directional coupler feeding the transceiver. The detected coherent emissions are processed with a matched filter to determine presence of a target object within the portal.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/628,824 filed on Sep. 27, 2012 and claims priority to U.S.Provisional Application No. 61/540,851, filed on Sep. 29, 2011 and U.S.Provisional Application No. 61/566,330 filed on Dec. 2, 2011. The entireteachings of the above application(s) are incorporated herein byreference.

BACKGROUND

This application relates to chemical analysis and more particularly tosystems and methods that use nuclear magnetic resonance.

It is known that an atom with more than one unpaired nuclear particle(protons or neutrons) will have a charge distribution which results inan electric quadrupole moment. Allowed nuclear energy levels are shiftedunequally due to the interaction of the nuclear charge with an electricfield gradient supplied by the non-uniform distribution electron density(e.g. from bonding electrons) and/or surrounding ions. This so-calledNuclear Quadrupole Resonance (NQR) effect results when transitions areinduced between these nuclear levels by an externally applied radiofrequency (RF) field. This electromagnetic field thus induces a magneticresonance, unique to each material, without using a magnet. A typicallyNQR detection system consists of a radio frequency (RF) power source, anemitter to produce the electromagnetic excitation field, and a detectorcircuit which monitors for a RF NQR response coming from the objectbeing analyzed.

NQR has a number of practical uses, such as the detection of land mines,or of narcotics or explosives concealed in luggage, or remote monitoringof fluid levels such as in oil wells.

SUMMARY

In a first aspect, a technique for detecting a substance uses one ormore conductive surfaces to define a space that is to be monitored. Twoor more wire loops are disposed within the space typically adjacent theconductive surfaces. The wire loops are each individually electricallyterminated in a preferred arrangement; alternatively, they can bearranged as balanced transmission lines. The wire loops are then drivenwith a radio frequency (RF) transmitter to create a time varyingelectromagnetic field within the defined space. The wire loops are, inone arrangement, individually electrically terminated through arespective resistance to a reference point, such as a ground voltagereference point.

The wire loops are connected to the transmitter via a directionalcoupler or in a similar fashion that avoids the use of ferrite materialthat might otherwise introduce nonlinearities in the system. The timevarying electromagnetic field stimulates nuclear quadrupole resonance inany material with an electric quadrupole moment located within the spaceto cause the material to emit coherent RF emissions. These RF emissionsare then detected using the same directional coupler through which thetransmitter is connected. The received emissions are then furtherprocessed to determine characteristics of the substance, such as bydetecting their amplitude, phase and/or frequency.

The NQR response for a given material is characterized as behavingaccording to the Rabi formulation that predicts a likelihood that thestimulated emission is either in the ground state or the excited state.We have realized that if the resonant frequency for a particularmaterial of interest is known, the power incident on the material isknown, and the exication signal is known (such as a chirp), the NQRresponse can be characterized. Thus the emissions can be continuouslyprocessed using a suitable matched filter to optimize detection.

It can also be discerned that a deterministic phase relationship existsbetween the reference and the emitted signal that depends on thecircuitry used to generate the two. In other words, the phase differencebetween the reference and the signal to be detected should account forpath differences in the circuits used to generate the two differentsignals. A criterion can then be set up to accept or reject a potentialauthentic NQR response signal based upon how close the measured trackedphase matches the theoretical expected phase.

In one particular arrangement the conductive surfaces are configured asa generally rectangular portal of convenient size, such as large enoughto permit a person to walk through. In this arrangement, one or morewire loops are disposed adjacent a first vertical conductive surface andone or more wireless are also disposed along a second opposite verticalconductive surface. If multiple wires are disposed adjacent a givensurface they can be driven with alternating polarities of RF signals.

In another arrangement, a single conductive surface can be disposed suchas in a floor or ceiling to define the space. In this example, it wouldbe typical for many wire loops to be disposed in the floor or ceilingadjacent the conductive surface but remaining within the space, againwith alternating polarities.

The emitted RF signal will optionally take the form of a frequencystepped or chirped signal centered about a specific radio frequency thatis known to be related to the NQR of a substance of interest. If asystem is to detect multiple substances of interest therefore, it willbe advantageous to emit multiple such signals centered around differentcarrier frequencies that correspond to resonances of the materials ofinterest. Thus corresponding detector will also detect coherentemissions such as with the corresponding number of RF filters.

In still other arrangements, a single RF transmitter and receiver can beused to operate multiple portals. In this arrangement, the differentportals each have their respective sets of wire loops. These wire loopsin the different portals are driven with orthogonal modulated RFsignals, such as by using Code Division Multiple Access (CDMA). Thecorresponding orthogonal demodulation process is implemented on thereceiving end.

Detection performance can be improved by determining a referenceemission when the portal is empty. This empty portal response iscompared to signal(s) detected when a substance is placed in the portal.However the comparison is not direct; in a preferred arrangement,complex-valued reference signal points are averaged to determine a startpoint and a stop point of a reference line that extends from a beginningsweep amplitude and frequency to an ending sweep amplitude andfrequency. The signal points of a detected emission from an object arethen similarly average to determine a start point and stop point. Adifference is then determined that is taken as a difference between thesignal and these reference lines as the detected emission.

BRIEF DESCRIPTION OF THE DRAWINGS

The description below refers to the accompanying drawings, of which:

FIG. 1 is a theoretical plot of continuous NQR coherent emissions fromsodium nitrate that result from a chirp excitation signal.

FIG. 2 is an actual measured response for sodium nitrate.

FIGS. 3A, 3B and 3C illustrate a loop-exicted space defining a cavityportal; the excitation is with individual resistor terminations.

FIGS. 3D, 3E and 3F illustrate another excited portal arrangement usingbalanced transmission lines excitation.

FIGS. 4A and 4B show a conducting half space layer placed on a floor.

FIG. 5 is a general block diagram of the system.

FIG. 6 is a more detailed view of processing the detected emissions.

FIG. 7 is a pictorial representation of one aspect of the processing.

FIG. 8 is a general block diagram showing signal paths.

FIG. 9 is a single portal multiple frequency block diagram.

FIG. 10 illustrates a multiple-portal, multiple frequency system.

FIG. 11 is a more detailed view of the multiple portal, multiplefrequency band system.

DETAILED DESCRIPTION OF AN EMBODIMENT

Historically, systems that make use of the Nuclear Quadrupole Resonance(NQR) effect to detect substances have used a large pulsed radiofrequency (RF) magnetic field followed by detection of a weak RF field.These fields are typically in the 1 MegaHertz (MHz) range. As a result,most prior existing NQR systems require high power, are large and bulky,and suffer from low sensitivity. The enhanced NQR detection systemdescribed will have one or more distinguishing characteristics.

Rabi Formulation to Characterize Continuous System Response

A formulation known as the Rabi formulation characterizes the responseof an atom to an applied harmonic field, when the applied frequency isclose to the atom's natural frequency. A simple approach is through atwo-energy level approximation, in which one only treats two energylevels of the atom in question. No atom with only two energy levelsexists in reality, but a transition between, for example, two hyperfinestates in an atom can be treated, to first approximation, as if onlythose two levels existed, assuming the drive is not too far offresonance.

Thus the NQR of a substance can be characterized using the general Rabiformulation in which the nucleus is assumed to oscillate between state 1(a ground state) and state 2 (an exicted state) under the influence ofthe time-dependent incident electromagnetic field. This implies that thenucleus alternatively absorbs energy from the incident field and emitscoherent energy induced by the incident field. The phenomenology isexpressed by Rabi's equations below (Equations 1 and 2).

$\begin{matrix}{{P_{1}(t)} = {1 - {P_{2}(t)}}} & (1) \\{{{P_{2}(t)} = {\frac{\gamma^{2}}{\gamma^{2} + {\left( {\omega - \omega_{NQR}} \right)^{2}/4}}{{SIN}^{2}\left( {\Omega \; t} \right)}}}{\Omega = \left\lbrack {\gamma^{2} + {\left( {\omega - \omega_{NQR}} \right)^{2}/4}} \right\rbrack^{1/2}}} & (2)\end{matrix}$

where P1 is the probability that the nucleus is in the ground state andP2 is the probability that the nucleus is in the excited state. 4γ isthe half power width. The SIN term in Equation 2 expresses the periodicnature of the emissions.

Using these Rabi formulations (Equations 1 and 2), if the NQR resonantfrequency, ω_(NQR), and variations in the power incident on a materialare known, a matched filter can be determined to optimized signaldetection.

For the case of detecting Sodium Nitrite, a material with known NQRfrequencies, the NQR signal response can be predicted assuming, forexample, that the incident field is a chirp waveform. The chirpinstantaneous frequency is given by Equation 3:

ω_(INSTANTANEOUS) =F _(START)2π+2π(BW/T)t   (3)

For the known Sodium Nitrite NQR frequency at 3607 kHz, the followingvalues are applicable:

-   -   BW=40 kHz    -   T=1 sec    -   4γ=100 Hz

The signal response 100 is estimated by convolving the chirp waveformwith the inverse Fourier transform of Equation 2.

FIG. 1 shows simulation results of the sodium nitrite NQR resonanceusing Equation 2. The pulsed RF represents the periodic coherentemissions centered at 3607 KHz. Note the estimated four energy peaks,102, 104, 106 and 108. FIG. 2 illustrates actual measured data from asodium nitrate sample. Note the energy peaks 202, 204, 206, 208, 210correspond more or less, but not exactly, to the theoretical model. Thisdifference is typically acceptable, as the system can store templates ofactual response measurements for different materials.

A matched filter is then used to coherently integrate all the pulsedemissions as part of the detection process.

Conductive Surfaces Define a Space

In a practical implementation, one or more conductive surfaces arearranged to define a space that is to be monitored such as for accesscontrol. FIGS. 3A, 3B and 3C illustrate one such cavity type arrangementwhere a generally rectangular portal 300 is defined by four conductivewalls 302-1, 302-2, 302-3, 302-4. Two or more wire loops 306-1, 306-2are disposed within the space, typically adjacent selected ones of theconductive surfaces 302. The wire loops 306 are each individuallyelectrically terminated through a resistance 310 to the respectiveconductive wall(s) in this arrangement. A coaxial cable connector 308-1,308-2 provides connection to the radio frequency (RF) transmitter andreceiver. The conductive walls 302 define the space within which auniform electromagnetic field can be maintained by the wire loopradiators while at the same time protecting the space from outsidedisturbances.

FIGS. 3D, 3E and 3F show another possible arrangement of the wire loops.There, the wire loops 316 are still disposed within the cavity 300.However, they are implemented as a balanced transmission line drivingtwo segments 318-1, 318-2 through a balun 328 with the two segments318-1, 318-2 having a resistance 320 disposed at their mid-point.

In another arrangement, the space to be monitored is defined as aconductive half-space 410. A system of wire loops 410 providesexcitation to such a conductive half space 400, such defined by a metalsurface 402 embedded in a floor, as shown in FIGS. 4A and 4B. The halfspace 400 can be a corridor or large open public area. In theillustration of FIGS. 4A and 4B, the loops 410 are individually fed bycoax feeds 408, and terminated by resistors 412. The coax feeds 408 mayhave alternating polarities, as shown. The excitation loop(s) layer andthe conducting half space layer can comprise a composite flexiblecarpet, in one example.

System Hardware Components

The preferred embodiment of the NQR electronics is shown in FIG. 5 for asingle portal, single frequency band. A laptop computer 500 or otherdata processor or digital signal processor controls a transceiver 502.The transceiver 502 generates a transmit waveform, Tx, and receivesreceive signal Sig and reference signal Ref. The transmit signal Tx isfed through a power amplifier 508 and attenuator 510 to a firstdirectional coupler A 506. A first output of directional coupler A 506is fed to a second directional coupler B 504. The directional coupler B504 then feeds the wire loops in the portal 300 or 200 to create a timevarying electromagnetic field within the space. The use of directionalcouplers 504, 506 that do not incorporate any ferrite material ispreferred, to avoid introducing nonlinearities in the system.

A baseband digital source 502 generates the chirp or stepped waveformunder control of the computer 500. This waveform is amplified andexcites the portal 300 or 400, creating a field which envelopes a personwalking through. If explosives are being carried by the person, thecoherent emissions are reflected through directional coupler (B) 504 atthe portal 300, 400 and fed to the transceiver signal input (515). Thefunctionality of each component of the block diagram of FIG. 5 istherefore as follows:

-   -   Laptop Computer (500): Executes a detection algorithm, such as        by using a stored computer program, processes raw data from the        transceiver, and outputs the NQR response. Could also be a        digital signal processor or other suitable machine.    -   Transceiver (502): Generates the input waveform and handles the        reference and coherent emission returns.    -   Power Amplifier (508): Amplifies the signal in order to excite        the portal.    -   15 dB Directional Coupler (A) (506): Provides a reference for        the system which is fed back into the transceiver.    -   15 dB Directional Coupler (B) (504): Feeds coherent emissions        reflected from the portal back into the transceiver.    -   Portal (300 or 400): Field detector.    -   Attenuators (510, 512, 514): Control power levels necessary for        the power amp 508 and transceiver 502.

In operation a “baseline” signal using an empty portal is continuouslyrecorded by the computer 500. As described in more detail below, thebaseline signal is then differentially combined with the signal acquiredfrom the person or other object in the portal.

System Software Components

Waveform Generation

The material detection system requires an input waveform which iscreated and/or stored by the computer 500 and fed into the transceiver502 to generate the transmit waveform Tx. The transmit waveforms ofinterest are 1) a Chirp Waveform and 2) a Stepped Frequency Waveform,Equations 4 and 5 respectively.

The chirp waveform is generated according to:

sin(F_(start) 2π t_(l)+π(Δ/T)t_(l) ²)   (4)

-   -   Δ=F_(stop)−F_(start)    -   Δ=40 kHz    -   T=(Dwell Interval)×400=1 sec    -   Dwell Interval=2.5×10⁻³ sec    -   t_(l)=l/sample rate 1≦l≦(sample rate×T)

The stepped frequency waveform can be given by:

sin(F_(N) 2π t_(l))   (5)

-   -   401 Frequencies within a 40 kHz Band    -   400 intervals, (40×10³/400)=100 Hz steps    -   Every Dwell Interval=2.5×10⁻³ sec

Step F_(N)→F_(N+1)

-   -   F₁=F_(start), F₄₀₁=F_(stop)    -   Δ=F_(stop)−F_(start)    -   Δ=40 kHz    -   T=(Dwell Interval)×400=1 sec    -   Dwell Interval=2.5×10⁻³ sec    -   t_(l)=l/sample rate 1≦l≦(sample rate×T)

The use of ferrite-free directional couplers permits the detection ofstimulated emissions that are as small as 10⁻⁸ to 10⁻¹⁰ of the transmitpower incident on the material.

Detection Processing

FIG. 6 is a flow diagram of the receive processing implemented orcontrolled by the computer 500. It should be understood that thesefunctions can be carried out entirely in software, or in special purposedigital signal processing hardware, or a combination of both. Thegeneral idea is to take a set of measurements with an empty portal 300,400, and process those along with a set of prior measurements taken withthe material of interest in the portal 300, 400.

Responses from the portal are processed as follows.

In a first step 602-1, a complex-valued (I and Q) reference signal isobtained at the Ref input of the transmitter 502 and converted todigital data through an Analog to Digital converter (ADC). The signalport (Sig) provides a complex-valued signal at the same time. S21(Sig/Ref) is then determined in step 604.

Two data runs are then performed—one with material of interest locatedin the portal (step 606-1) and one run with the empty portal (step606-2).

A corresponding linear end point decomposition (steps 608-1 and 608-2)is then performed on each measurement. This decomposition is describedin more detail in connection with FIG. 7 below.

Next, a cancellation algorithm is applied in step 610 to remove theeffect of the portal on the measurement.

Finally, a phase filtering operation (step 614) is applied to removeartifacts of phase differences in the reference and signal paths, toobtain the response that is considered the response due only to NQR ofthe material.

More particularly, steps 608-1 and 608-2 normalize raw data receivedfrom the portal, V_(SIG), with material in it, it using reference datareceived from an empty portal, V_(EMPTY), previously collected. V_(SIG),data received from the portal with material in it, and V_(EMPTY), datafrom the portal without material in it, are complex functions. FIG. 7shows such an example V_(SIG) and V_(EMPTY) plotted on the I and Qcomplex-valued plane. Each data point is represented as a vectormagnitude and frequency (angle). The responses V_(EMPTY) and V_(SIG)thus manifest as a moving vector in the complex plane.

Average segments, L_(SIG) and L_(EMPTY), are then developed and thencompared to the measured values. More particularly, respective start andstop points of L_(SIG) and L_(EMPTY) are obtained by averaging V_(SIG)and V_(EMPTY) over a small percentage of the input sweep signal centeredat the endpoints of each respective segment. L_(SIG) and L_(EMPTY) arestraight line segments.

V _(OUT)=(V _(SIG) −L _(SIG))−(V _(EMPTY) −L _(EMPTY))   (6)

The intermediate output, V_(OUT) (Equation 6) is then applied to a phasematching step and then the coherent pulse train matched filter for thefinal NQR output, (example coherent pulse trains were shown in FIGS. 1and 2.)

Phase Matching

Since the NQR signal of interest is derived from the stimulated emissionof the excited states of the nucleus, there is also a deterministicphase relationship between the reference and the NQR signal of interest.

The phase difference between the reference and the NQR signal can thusbe determined by considering the path differences of the reference (pathC) and the signal (path B+path A) through the system. These differencesdepend upon the delay in paths A, B, and, C as depicted in FIG. 8.

The end result of the above relationships is that V_(OUT) (Equation 6),which is calculated from reference and signal measurements, representsthe actual stimulated emission output (NQR signal) and has adeterministic phase (all phases are measured relative to the referencechannel).

A criterion can be set to accept or reject a potential authentic NQRsignal, based on how close the measured phase tracks the theoreticalphase. A library of expected responses from a set of materials is thedeveloped from actual measurements. The library may include responsesunder different conditions known to affect NQR such as temperature,humidity, etc.

Decision/Matched Filtering

As alluded to above, a final step is to match the resultant responseagainst one or more known response(s) to determine the type of materialdetected. This matching process can match against a library of templatesof previously detected responses (such as FIG. 2) or theoreticalexpected responses (such as FIG. 1). The matching may compare amplitudepeaks and corresponding phases, or may be a more mathematically robustmatched filter.

Single Portal, Multiple Frequency Bands Implementation

In order to handle multiple frequency bands simultaneously for a singleportal configuration, Frequency-Division Multiple Access (FDMA) isemployed. With this approach, multiple transmit signals, such asmultiple chirp signals, are generated at different RF carriers. Thereceiver can then use a corresponding set of frequency domain frequencyfilters which are accessed within the transceiver.

The block diagram of this single portal, multiple frequency bandsimplementation is shown in FIG. 9.

Multiple Portals, Multiple Frequency Bands Implementation

It is also possible to run a multiple portal, multiple frequency bandsystem with some modifications to the single portal, single frequencyband system architecture. The general block diagram for a multipleportal, multiple frequency band system is shown in FIG. 10; themodification is to use Code Division Multiple Access (CDMA) or someother orthogonal modulation scheme to separate the signals associatedwith different portals.

Thus, in order to simultaneously handle multiple portals and multiplefrequency bands, Code-Division Multiple Access (CDMA) andFrequency-Division Multiple Access (FDMA) are both employed. CDMAhandles multiple portals simultaneously and then filters the informationfrom each of the multiple portals through de-coding. These de-codedresponses are then fed through FDMA filters which frequency divides thesimultaneous frequency band information from each portal.

Ultimately, the sophisticated waveform input (N-Portals, m-FrequencyBands) that is fed into the material detection system is able to handleN-Portals and m-Frequency Bands simultaneously while giving filteredoutput that is portal and frequency band binned so that the separateresponses are of value. A high level block diagram of the multipleportals, multiple frequency bands implementation is shown in FIG. 10,and in more detail in FIG. 11.

While this invention has been particularly shown and described withreferences to example embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A method for detecting a substance comprising:disposing a first and second planar conductive surface in parallel withone another and spaced apart from one another to define a space betweenthem; disposing a first wire conductor within the space, the first wireconductor having a first end and a second end, the second endelectrically terminated through a resistance to the first conductivesurface; disposing a second wire conductor within the space, the secondwire conductor having a first end and a second end, the second endelectrically terminated through a resistance to the second conductivesurface; driving the first and second wire conductors with a basebandchirped radio frequency transmitter through a directional coupler tocreate a time varying electromagnetic field within the space; receivingresulting coherent radio frequency emissions through the samedirectional coupler through which the transmitter is coupled; and isprocessing the coherent radio frequency emissions with a radio frequencyfiltering step to determine characteristics of a substance in the space.2. The method of claim 1 additionally comprising: disposing a third andfourth conductive surface adjacent the first and second conductivesurface to define the space as a generally rectangular portal.
 3. Themethod of claim 1 wherein the first wire conductor is disposed withinthe space adjacent the first conductive surface, and the second wireconductor is disposed within the space adjacent the second conductivesurface.
 4. The method of claim 1 additionally comprising: disposingmultiple wire conductors adjacent one another and adjacent a selectedconductive surface.
 5. The method of claim 4 with adjacent wireconductors driven with radio frequency signals of alternatingpolarities.
 6. The method of claim 1 additionally comprising: detectinga portion of transmitter power with a directional coupler, to provide areference signal, and where the step of processing the coherentemissions further uses the reference signal to determine characteristicsof the substance.
 7. The method of claim 1 wherein the stimulatedemissions are limited to between about 10⁻⁸ and 10⁻¹³ of the transmittedenergy incident on the space.
 8. The method of claim 6 wherein thedirectional coupler has a linear response.
 9. The method of claim 6wherein the directional coupler is free of ferrite material that wouldotherwise introduce non-linearities.
 10. An apparatus for detecting asubstance comprising: a first planar conductive surface; a second planarconductive surface; the first and second planar conductive surfacesspaced apart from each other and disposed in parallel with each other todefine a space between the first and second planar conductive surfaces;a first wire conductor disposed adjacent the first planar conductivesurface and disposed within the space, the first wire conductor having afirst end and a second end, the second end electrically terminatedthrough a resistance to the first conductive surface; a second wireconductor disposed adjacent the second planar conductive surface anddisposed within the space, the second wire conductor having a first endand a second end, the second end electrically terminated through aresistance to the second conductive surface; a chirped radio frequencytransmitter coupled to the second end of the first wire conductor and tothe second end of the second wire conductor, to create a time varyingelectromagnetic field within the cavity, the time varyingelectromagnetic field stimulating continuous transitions between twoenergy states in the nucleus of a substance disposed within the space;and a radio frequency receiver for receiving resulting coherent radiofrequency emissions from the substance.
 11. The apparatus of claim 10additionally comprising: a third conductive surface extending betweenthe first and second conductive surfaces; a fourth conductive surfaceextending between the first and second conductive surfaces at a locationopposite the third conductive surface; wherein the first end of thefirst conductor is located adjacent the third conductive surface; thesecond end of the first conductor is located adjacent the fourthconductive surface; the first end of the second conductor is locatedadjacent the third conductive surface; and the second end of the firstconductor is located adjacent the fourth conductive surface.
 12. Theapparatus of claim 11 wherein multiple wire conductors are disposedadjacent one another and adjacent a same selected one of the conductivesurfaces.
 13. The apparatus of claim 12 wherein the multiple wireconductors are disposed with adjacent ones of the wire conductors drivenwith radio frequency signals of alternating polarities.
 14. Theapparatus of claim 10 additionally comprising: a first directionalcoupler, connected between the transmitter and the first and second wireconductors.
 15. The apparatus of claim 14 additionally comprising: asecond directional coupler, connected to detect a portion of transmitterpower as a reference signal.
 16. The apparatus of claim 10 whereinmultiple wire conductors are disposed adjacent one another and adjacenta same selected one of the conductive surfaces.